1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit or a liquid crystal device by using photolithography. The projection exposure apparatus projects and transfers a pattern (including a plurality of different types of patterns) formed on a reticle (mask) onto a substrate such as a wafer via a projection optical system.
Along with the recent increase in the packing density of semiconductor devices, it is becoming indispensable to reduce the aberrations and distortion of the projection optical system in order to transfer a pattern having a high packing density onto a substrate with high accuracy. Note that the aberrations of the projection optical system include not only aberrations in design values but also aberrations due to, for example, fabrication errors at the fabrication stage, those due to the environment (ambient pressure and temperature) in which the exposure apparatus is installed, those which change over time, and those due to exposure heat.
Under these circumstances, Japanese Patent Laid-Open Nos. 2000-357651, 2004-347821, and 2006-113414 propose techniques for reducing (correcting) aberrations and distortion in projection optical systems. For example, Japanese Patent Laid-Open No. 2000-357651 discloses a technique of correcting, for example, magnification aberration, third-order distortion, spherical aberration, and field curvature by driving an optical element which forms the projection optical system or the reticle in the optical axis direction of the projection optical system. Japanese Patent Laid-Open No. 2004-347821 discloses a technique of correcting, for example, decentering distortion, on-axis coma, and local defocus astigmatism by tilting an optical element which forms the projection optical system or the reticle with respect to a plane perpendicular to the optical axis of the projection optical system. Japanese Patent Laid-Open No. 2006-113414 discloses a technique of correcting the imaging performance of the projection optical system by deforming the shape of an optical element which forms the projection optical system.
Unfortunately, unintended aberrations often occur in the prior art due to a driving error when driving the optical element or reticle. This makes it impossible to sufficiently reduce (correct) the aberrations and distortion of the projection optical system.
For example, when the optical element (or reticle) is driven in the optical axis direction of the projection optical system, only aberrations symmetrical about the optical axis, such as magnification aberration, third-order distortion, spherical aberration, and field curvature, are supposed to change as long as there is no driving error, as shown in FIG. 3A. However, in practice, it is difficult to drive the optical element exactly in the optical axis direction, so the optical element is often driven obliquely with respect to the optical axis, as shown in FIG. 3B. In this case, aberrations asymmetrical about the optical axis, such as decentering distortion, on-axis coma, and local defocus astigmatism, change despite the user's intention.
Also, when the optical element (or reticle) is tilted with respect to a plane perpendicular to the optical axis of the projection optical system, aberrations asymmetrical about the optical axis, such as decentering distortion, on-axis coma, local defocus astigmatism, can be corrected to target values as long as there is no driving error, as shown in FIG. 4A. However, in practice, the optical element is slightly, parallelly driven in a direction perpendicular to the optical axis, as shown in FIG. 4B, resulting in deviations of the aberrations asymmetrical about the optical axis from their target values.